Characterization of Cerium and Oxygen Atoms in Free Clusters of Cerium Oxide by X-ray Absorption Spectroscopy
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X-ray absorption spectroscopy of size-selected cerium oxide cluster ions, Ce3O4–7 +, has been carried out by fragment-ion-yield measurement. X-ray absorption spectra measured in the Ce M4-edge and the O K-edge regions provided novel experimental data for chemical analysis of the constituent atoms and for discussion of geometric structures. The spectra near the Ce M4-edge exhibited a clear main peak and subtle substructures. Composition dependence of the spectra indicated that the oxidation state of Ce atoms evolves from + 3 to + 4 as an O atom is introduced one by one. On the other hand, the O K-edge spectra consisted of a main peak at ca. 532 eV and two additional peaks in the pre-edge region, where the intensities of the pre-edge peaks were significantly dependent on Ce:O composition. These pre-edge peaks suggested that the geometric structures of Ce3O5–7 + retain the “framework” of Ce3O4 + with “peripheral” O atoms bound on-top to the Ce atoms; coexistence of a structural isomer was suggested for Ce3O6 +, indicating presence of a molecular oxygen with Ce3O4 +−O2 geometry.
KeywordsCerium oxide clusters X-ray absorption spectroscopy Chemical analysis Oxidation states Composition dependence
We thank Professors Hiroshi Kumigashira, Koji Horiba, and Makoto Minohara for their technical support in the beamline BL-2B, KEK-PF, and Professor Kenta Amemiya in the beamline BL-7A. We are grateful to Professor Masahiko Ichihashi for informing us about his computational results prior to publication. We also thank Dr. Kazuhiro Egashira for his help in preparatory experiments. This work has been supported by the Special Cluster Research Project of Genesis Research Institute, Inc., and was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2014G091 and 2016G183).
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